In ordinary domestic sewer pipe systems and more often in industrial sewer networks, it frequently happens that noxious, toxic and even explosive gases and liquids are present and free to circulate through the sewer pipes. This poses a definite danger to cleaning and maintenance personnel. To cope with the problem, the prior art has developed various seals and traps to contain dangerous fluids in one location, so that they may be safely removed.
For example, Canadian Patent 918,531 teaches the use of a tank filled with water which acts as a seal to prevent the discharge of explosive sewer vapors into the surrounding air. However, this invention is designed for installation at the top of a manhole and would not prevent the circulation of such vapors in the sewer system underneath, which is where they form. Another Canadian Pat. No. 97,500, is similar to the present invention as disclosed below. Even so, this patent has the disadvantage of being relatively easily dislogable by a counter-flow of sewer fluids which sometimes occurs in a multiple manhole sewer network.
Other prior art generally teaches the use of devices which are more or less permanently installed. The fact that such devices are difficult to remove constitutes another disadvantage because they obstruct either a manhole or its catch-basin, or the sewer pipes themselves. Thus, modern maintenance equipment, such as closed-circuit t.v. cameras and cleaning machines adapted to be pulled through a sewer pipe by a cable, cannot be properly used.